An automated analyzer for clinical laboratory test dispenses fixed amounts of a specimen and a reagent to cause a reaction, measures the absorbance of a reaction solution for a fixed period of time, and obtains a laboratory test value (the concentration or activity value) of a substance to be measured and the like on the basis of a measurement result.
A rate method that is one of measurement methods for clinical laboratory test is mainly used to measure the activity value of an enzyme component contained in a specimen. The measurement method includes adding a fixed amount of a substrate as a reagent and measuring an element that changes when an enzyme consumes the substrate. Generally, a reagent contains a sufficient amount of a substrate. For this reason, if a reaction between a specimen and a reagent is normally being carried out, the reaction is generally such that absorbance changes linearly with time in increments of a fixed amount. In the rate method, the activity value of a substrate to be measured is obtained from the slope of such a straight line (reaction rate).
However, there may be a curve part called a lag phase before a reaction rate becomes constant (linear) due to factors such as a measurement item, the concentration of a sample, stirring status, and reaction temperature. In the case of, e.g., a high-activity sample, a reaction may progress fast. In this case, the amount of a substrate in a reagent may become insufficient during measurement, and a reaction rate in a second half may be inconstant. That is, the reaction rate in the second half may change non-linearly (in a curved manner).
As described above, a curve part may appear in reaction curve data at the beginning and at a later part of a reaction. For this reason, a conventional device adopts a process of setting a time range used for laboratory test value conversion and an absorbance limit value (Abs limit) for each item. Some of conventional devices have a function for checking linearity called a linearity check. In a linearity check, the difference between amounts of change in absorbance in a first half and a second half of a fixed photometric range is calculated, and change in absorbance is determined to be not linear if the difference is above a designated linearity check value. If the change in absorbance is determined to be not linear, a conventional device makes a notification of “abnormality.”
Methods for determining a time range (line range) used for laboratory test value conversion include the methods disclosed in Patent Literatures 1 and 2. Patent Literature 1 discloses a process of calculating an absorbance difference between each pair of adjacent photometric points, setting as a reference point a range between one having the largest absorbance difference (dmax) among the pairs, and determining as a line range sections having absorbance differences that are not less than 80% of the absorbance difference (dmax) of the reference point among sections within a measurement range straddling the reference point. Patent Literature 2 discloses a process of setting the width of a range used for conversion to a fixed width, calculating a coefficient of correlation among absorbance corresponding to the range by the least squares method while shifting the range by one point at a time, starting from a top photometric point, and determining a range with the best correlation as a line range.
In recent years, there has been a need to report a result of measuring patient samples as soon as possible in the field of clinical laboratory testing in hospitals. The reaction time period during measurement of an automated analyzer is about 10 minutes. However, simply shortening a measurement time period prevents acquisition of precise results. For example, if measurement is ended at a time when a reaction is incomplete, and a laboratory test value is measured from the absorbance at the time, a precise result is not obtained. To cope with this, it is conceivable to predict the absorbance at the end of a reaction and convert the absorbance to a laboratory test value. As a method for predicting the absorbance at the end of a reaction, Patent Literature 3 is publicly known. A process of approximating reaction curve data using an approximation formula indicated by (Expression 1) and obtaining, in the case of a rate method, the slope of a straight line from two predetermined points or one predetermined point after differentiation of the approximation formula with respect to time is disclosed in Patent Literature 3. Note that, in (Expression 1), t represents a time, y represents absorbance, and that A, B, and K are parameters.y=A+(B−A)/eKt  (Expression 1)